The present invention relates to a method of transmitting signaling data. It can be used in the telephony art in particular to interconnect two exchanges, in particular two private exchanges.
Constituting transmission channels involving, in principle, physically, temporally or functionally reserving means for sending data and physically, temporally or functionally reserving complementary means for sending signaling signals is known in the telephony art. Signaling signals, or signaling in general, organize(s) the transfer of data on the other means. Thus data channels or signaling channels can be physically different channels, pairs of telephone wires in a multi-pair cable. They can equally be frequency allocations in an overall frequency band. Finally, from the functional point of view, they can be messages sent on a channel but whose destination is sometimes one person and sometimes another person, depending on signaling data contained in the message.
It is usual to distinguish between homogeneous access and hybrid access. With homogeneous access the channels used for data are of the same type as the channels used to send signaling. The invention is more particularly directed to hybrid access in which the channel types are different, although the invention can equally be adapted to suit homogeneous access.
If two exchanges are interconnected, it is necessary for them to use identical transaction protocols on their common communication channels. To this end, the ISDN standard relating to integrated services digital networks (ISDN) defines a protocol offering relatively high performance. Consequently, since the introduction of the ISDN standard, new exchanges have conformed to it.
However, in extant public networks in particular, some communication channels do not conform to the ISDN standard. This is because they were designed a long time ago, because their objectives are different to those addressed by the standard, or because their modes of use are then more efficient. Examples of such networks are Ethernet networks and QSig-GF networks.
A problem therefore arises because the equipment is not homogeneous, comprising exchanges that comply with the ISDN standard, on the one hand, and exchanges that do not comply with it, on the other hand. The problem lies in the fact that the ISDN standard defines a protocol for sending signaling signals relating to telephone calls to be set up or modified. In practice it is the definition of the protocol which causes the problem. It cannot be implemented in networks which do not conform to the ISDN standard. The ISDN protocol provides service access point identifier messages, comprising SAPI S messages for signaling and SAPI P messages for sending packets. When the exchange equipment is manufactured the problem therefore arises of the inability of the exchange to route calls conforming to its standardized protocol over a network of a type other that conforming to the standard, and in particular a network which does not accept signaling messages.
The invention solves this problem by converting the signaling data produced by the exchange in the standard ISDN format into signaling data in a format accepted by the channel. The format accepted by the channel includes the addition to the signaling data of information indicating that it is signaling data. Because the channel that is to be used does not make any distinction, a protocol that the invention determines in advance is used to advise it that the signaling messages are in fact signaling messages.
In more concrete terms, given the random nature of the requirement to send signaling messages (which are sent only when a call is set up between two parties or if the call conditions are modified), it would appear necessary to have a channel open at all times to convey the signaling messages. However, a channel that is open at all times consumes a great amount of resources, in particular if it does not carry a great amount of information. One kind of channel that is open at all times well known to the skilled person is an Ethernet network. An Ethernet network is designed to route packets of information fed to it. However, although it corresponds effectively to the requirements stated by exchange proprietors, from this point of view of permanent availability, an Ethernet network does not conform to the ISDN standard. The treatment to encapsulate data in order to transport it on Ethernet networks uses software that conforms to the UDP-IP (User Datagram Protocol—Internet Protocol) standard, for example. The UDP-IP standard is not structured like the ISDN standard and is not compatible with it. In particular, with the UDP-IP standard, the sending of information packets is not assured and their order of arrival is even less assured.
To solve this problem, the invention adds information representing the order of the packets to signaling packets sent over an Ethernet network. When a packet is received, an acknowledgment is sent to the sender. The acknowledgment includes the number of the last packet received. This tells the sender which information packets have not been received and must be sent again.
In another example, the channel used to send the signaling messages is a standard QSig-GF channel, which does not conform to the ISDN standard either. Unlike an Ethernet network, which does not have any signaling channel at all, standard QSig-GF networks include signaling channels. However, their signaling channels are capable of carrying only SAPI S messages, not SAPI P messages. Consequently, none of the SAPI P information generated by the exchanges in the context of ISDN calls can be routed.
In this case, the invention exploits the existence in the QSig-GF protocol of a particular kind of availability referred to as a “FACILITY” message and enabling any type of information to be transmitted within a FACILITY message, whilst still complying with a form of encapsulation specific to the QSig-GF standard. The invention therefore starts by setting up a call with no B channel between the two exchanges connected by a QSig-GF link. This call with no B channel can then be used to enable the two exchanges to interchange FACILITY messages relating to the call on the D channel and therefore to encapsulate signaling messages with a header corresponding to the QSig-GF standard.